Theoretical and experimental research in the field of superconducting materials by thousands of researchers has led to the discovery of a handful of oxide compounds which become superconducting at relatively high temperatures (T.sub.C). i.e., within the range of 40-126 K. All of the known high temperature superconductors are oxides, and contain (1) copper and/or bismuth, (2) barium or other alkaline earths such as strontium or calcium, and (3) trivalent elements such as yttrium. Rare earth elements 57 to 71 (lanthanum to lutecium), thallium or bismuth are substituted for yttrium in some materials. Notable superconductors are as follows:
(1) oxide materials containing lanthanum, strontium and copper, bearing the formula La.sub.2-x Sr.sub.x CuO.sub.4, commonly referred to as L-S-C-O;
(2) oxide materials containing yttrium, barium and copper, bearing the formula YBa.sub.2 Cu.sub.3 O.sub.7-.delta., commonly referred to as 1-2-3 (rare earth elements can be substituted for yttrium, and the resulting compounds are also superconducting);
(3) oxide materials containing bismuth strontium, calcium and copper, bearing such formulas as Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8+x and commonly referred to as B-C-S-C-O;
(4) oxide materials containing thallium, barium, calcium and copper, bearing such formulas as Tl.sub.2 Ba.sub.2 CuO.sub.x, Tl.sub.2 CaBa.sub.2 Cu.sub.2 O.sub.x, Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x, Tl.sub.2 Ca.sub.3 Ba.sub.2 Cu.sub.4 O.sub.x, TlCaBa.sub.2 Cu.sub.2 O.sub.x, and TlCa.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x, commonly referred to as T-C-B-C-O; and
(5) oxide materials containing bismuth, barium, potassium and copper, bearing the formula Ba.sub.1-x K.sub.x BiO.sub.3, identified as B-K-B-O.
Each of these high temperature superconducting materials has drawbacks. For example:
La.sub.2-x Sr.sub.x CuO.sub.4 has a T.sub.C of only 40 K.
YBa.sub.2 Cu.sub.3 O.sub.7-.delta. has an oxygen content which varies with temperature, and is only superconducting when the oxygen content is high (.delta.&lt;0.4). A unique feature of the 1-2-3 compound is the variable oxygen stoichiometry, and rapid and reversible oxidation/reduction above 400.degree. C. Changing the oxygen content changes the superconducting transition temperature T.sub.C and also the normal state resistivity. The required high oxygen content is only reached in equilibrium at low temperatures (.about.400.degree. C.). At relatively high temperatures, the oxygen content of the crystal structure has an equilibrium value which is lower than the equilibrium value at lower temperatures. Temperature changes encountered during the processing of this material thus brings about changes in chemical composition and crystal structure, occurring slowly and nonuniformly. At the high temperatures needed for synthesis, grain growth and densification, the oxygen loss is severe, and is frequently accompanied by the formation of a liquid phase and the reordering of the crystal structure to segregate the material into two solid phases as the liquid phase resolidifies. The presence of two phases of different crystal structure and empirical composition is detrimental to the uniformity of atomic structure near grain boundaries and introduces certainty and nonuniformities in the critical temperature. In addition above about 700.degree. C., with loss of oxygen, the structure changes from orthorhombic to tetragonal. During cooling after synthesis, the structure changes back to orthorhombic as oxygen is absorbed. The existence of two phases gives rise to internal stresses in the material during cooling, causing twinning and its associated detrimental effects.
B-C-S-C-O has a soft flaky structure, while T-C-B-C-O contains highly toxic thallium. B-K-B-O is extremely sensitive to moisture and decomposes readily. In addition. B-K-B-O has a very low T.sub.C .apprxeq.30 K.
Materials (1), (2), (3) and (4) all contain copper as an essential constituent. The copper in these materials is in a highly oxidized state, with a formal valence greater than +2. This means that either some of the copper is in the rare trivalent state, or the oxygen is of a valency smaller than the normal -2.
The need for additional high temperature superconductors is great, not only to achieve superconductors with higher T.sub.C 's, but also to achieve superconductors with improved mechanical properties, stability and ease of processing. The discovery and study of new high temperature superconductors varying in composition and structure also provides the benefit of insight into the superconducting phenomenon, which will aid in finding a theoretical explanation. This will lead to a more systematic design of superconductors with needed properties.